Just to clarify the issue - the default setting for firefox spell checker is the curvy line which is ui.SpellCheckerUnderlineStyle value=5
But iceweasel can not display the curvy line for some reason, at least on my instalation. The key ui.SpellCheckerUnderlineStyle is not in prefs.js, because value 5 is applied by default. So I had to insert the key in order to change the underlining and have it working with values 1 to 4 which are displayed properly.